Field of the Invention
The present invention relates to a capacitive transducer used as an ultrasonic conversion element and a method of manufacturing the capacitive transducer.
Description of the Related Art
Up to now, a capacitive transducer (capacitive micromachined ultrasonic transducer (CMUT)) manufactured using micromachining technology has been studied as an alternative to a piezoelectric element. The CMUT can transmit and receive acoustic waves by means of vibrations of a vibration film, and can be easily provided with excellent broadband characteristics particularly in liquid. Note that, herein, the acoustic waves include waves called sonic waves, ultrasonic waves and photoacoustic waves. For example, the acoustic waves include photoacoustic waves that are generated inside of a subject by irradiating the inside of the subject with light (electromagnetic waves) such as visible light and infrared light. In actual use, a plurality of vibration films placed in a two-dimensional array is defined as one element, and a plurality of the elements is further arranged on a substrate, whereby a transducer is configured so as to achieve desired performance. In order to independently control each element, wiring electrodes respectively corresponding to the elements need to be formed. In this case, in order to reduce a structure size and the parasitic capacitance of each wiring electrode, it is desirable to use a through wiring that passes through the substrate. Meanwhile, in the case where the material of the through wiring is different from the material of the substrate, an end part of the through wiring may protrude from the substrate surface to deform or break through thin films located above the end part of the through wiring, due to a difference in thermal expansion coefficient in a high-temperature process after the formation of the through wiring.
U.S. Pat. No. 6836020 discloses a CMUT including a substrate made of silicon and a through wiring made of polycrystalline silicon. In this configuration, the through wiring made of polycrystalline silicon and the substrate made of silicon have approximately similar thermal expansion coefficient as each other, and hence a change in position of an end part of the through wiring relative to the substrate surface is small even at high temperature. Moreover, Japanese Patent Application Laid-Open No. 2007-215177 discloses a CMUT in which a glass substrate having a through wiring formed therein and another substrate are joined to each other. In this configuration, when thin films including a vibration film are formed on the other substrate, the thin films are not influenced by the through wiring. Moreover, Japanese Patent Application Laid-Open No. 2012-99518 discloses a through wiring structure configured using a concave part and a plurality of fine holes formed on the bottom surface of the concave part. In this two-stage wiring configuration, because the fine holes are small, a stress applied to thin films located above an end part of the through wiring is small even at high temperature.
However, in the case of the through wiring made of polycrystalline silicon in U.S. Pat. No. 6836020, because the resistivity of the polycrystalline silicon is high, it is far from easy to reduce the resistance of the through wiring. In the case of the joining method in Japanese Patent Application Laid-Open No. 2007-215177, a lower electrode is directly connected to an end part of the through wiring, and hence deformation of the lower electrode due to thermal deformation of the through wiring is unavoidable. Furthermore, in the case of the two-stage wiring configuration in Japanese Patent Application Laid-Open No. 2012-99518, the number of manufacturing steps is large, and a substrate area occupied by the through wiring is large. Hence, this configuration is not suitable for a reduction in size.